REDMOND, Wash. — Microsoft Corp. on Tuesday sent a letter to U.S. Attorney General Eric Holder calling on him to personally review a gag order that prevents the company and others from further discussing secret national security requests for customer emails, Skype calls and documents stored on its servers.

The letter follows Microsoft's motion in a secret court last month asking for permission to break out which requests were from law enforcement officials and which were from the National Security Agency. It hasn't received a response.